Time marches relentlessly forward for you and me; watch a movie in reverse, and you'll quickly see something is amiss. But from the point of view of a single, isolated particle, the passage of time looks the same in either direction. For instance, a movie of two particles scattering off of each other would look just as sensible in reverse – a concept known as time reversal symmetry.

Now the BaBar experiment at the Department of Energy's (DOE) SLAC National Accelerator Laboratory has made the first direct observation of a long-theorized exception to this rule.

Digging through nearly 10 years of data from billions of particle collisions, researchers found that certain particle types change into one another much more often in one way than they do in the other, a violation of time reversal symmetry and confirmation that some subatomic processes have a preferred direction of time.

Reported this week in the journal Physical Review Letters, the results are impressively robust, with a 1 in 10 tredecillion (10^43) or 14-sigma level of certainty – far more than needed to declare a discovery.

"It was exciting to design an experimental analysis that enabled us to observe, directly and unambiguously, the asymmetrical nature of time," said BaBar collaborator Fernando Martínez-Vidal, associate professor at the University of Valencia and member of the Instituto de Fisica Corpuscular (IFIC), who led the investigation. "This is a sophisticated analysis, the kind of experimental work that can only be done when an experiment is mature."